1. Field of the Invention
The present invention relates to an electric double layer capacitor and a battery. More specifically, the present invention relates to an electric double layer capacitor and a battery in which a cell is covered by an outer package in a decompression state.
2. Description of Related Art
Electric double layer capacitors have been conventionally used in practice and the structural characteristics thereof have recently been utilized to further decease their sizes and increase their capacities. For instance, considerations have been made for new uses of electric double layer capacitors, such as a driving power source for a cell motor of a vehicle in combination with a lead battery or an auxiliary power source in combination with a solar-battery.
Examples of the structure of electric double layer capacitors include the one shown in FIGS. 7 and 8.
The electric double layer capacitor shown in FIG. 7 includes a basic cell 55 which is constituted by a separator 51 of porous sheet type, a pair of polarized electrodes 52 and 52 of plate shape, a pair of collectors 53 and 53 of sheet shape, and gaskets 54 of frame shape. The pair of polarized electrodes 52 is disposed so as to sandwich and make contact with the separator 51 from both sides. The pair of collectors 53 is disposed so as to sandwich the polarized electrodes 52. That is, each of the pair of collectors 53 makes contact with the surface of the polarized electrodes 52 which is opposite the surface making contact with the separator 51. The gaskets 54 are disposed between the pair of collectors 53 so as to make contact with end surfaces of the polarized electrodes 52. The basic cell 55 contains an electrolyte sealed inside thereof.
In the electric double layer capacitor, a plurality of the above-mentioned basic cells 55 is laminated to form a multi-layered cell 56 shown in FIG. 8. In FIG. 8, an electrode plate 57 to which a lead terminal is connected is closely attached to the collectors 53 located at the outermost side in the laminated direction of the multi-layered cell 56, and the whole is covered by an outer package 58 in a decompression state (i.e., vacuum packed).
Since the electric double layer capacitor is covered by the outer package 58 in a decompression state, a compressive force is always applied to the capacitor from the outside due to atmospheric pressure. As a result, the degree of contact between the basic cells 55 and 55, and between the basic cell 55 and the electrode plate 57 having the lead terminal is enhanced to decrease the equivalent series resistance (ESR). Accordingly, the performance of the capacitor may be further improved and stabilized.
The need for an-electric double layer capacitor having an improved performance has recently been increasing and a further reduction of the ESR thereof is awaited. It has been discovered that if, for instance, a conductive film in which carbon is dispersed in an olefin resin is used for the collector 53 instead of a conventional conductive rubber in which carbon is dispersed in butyl rubber, the ESR of the collector 53 may be further reduced.
However, some of the above-mentioned conductive films have gas permeability coefficients higher than those of conventional ones, and electrolytes contained in the basic cells 55 may be gasified and leaked out from an interface 59 between the collectors 53 and the electrode plate 57 of the basic cells 55 which are located at the outermost side of the multi-layered cell 56 in some cases. If a gas were to leak out from the basic cell 55, the decompression state in the outer package 58 would no longer be maintained and the degree of contact between the basic cells 55 and 55, and between the basic cell 55 and the electrode plate 57 having a lead terminal would decrease. If the degree of contact were to decrease, the ESR thereof increases and the performance as a capacitor deteriorates and becomes unstable. This kind of problem is especially conspicuous under a high temperature atmosphere.
The present invention takes into consideration the above-mentioned circumstances, with an object of providing an electric double layer capacitor and a battery, which uses a material having high gas permeability constant for a collector in order to decrease the ESR, yet capable of preventing the increase of ESR due to a gas leak and, hence, electric performance thereof may be stabilized even under a high temperature atmosphere.